From a viewpoint of global environment protection, it has been studied that a fuel cell is utilized as a power supply of a motor which operates in place of an internal combustion engine of a vehicle, and that the vehicle is driven by the motor. It is not necessary for this fuel cell to use a fossil fuel having a resource depletion problem, and accordingly, the fuel cell never generates exhaust gas. Moreover, the fuel cell has excellent features in that noise is hardly generated, further, that it is also possible to enhance energy collection efficiency more than those of engines using other energies, and the like.
According to types of electrolytes for use, in the fuel cell, there are a polymer electrolyte type, a phosphoric acid type, a molten carbonate type, a solid oxide type, and the like. A polymer electrolyte fuel cell (PEFC) as one among them is a cell which uses, as an electrolyte, a polymer electrolyte membrane having proton exchangers in molecules, and utilizes the behavior that the polymer electrolyte membrane functions as a proton-conductive electrolyte when being made hydrous in a saturated manner. The polymer electrolyte fuel cell operates at relatively low temperature, and has high power generation efficiency. Moreover, the polymer electrolyte fuel cell is compact and lightweight as other utilities are, and accordingly, various applications which include mounting thereof on an electric vehicle are expected.
The above-described polymer electrolyte fuel cell has a fuel cell stack. The fuel cell stack is integrally configured by stacking a plurality of single cells, sandwiching both ends thereof by end flanges, and being pressurized and held by fastening bolts. Each single cell is composed of the polymer electrolyte membrane, and of an anode (hydrogen electrode) and a cathode (oxygen electrode) which are joined to both ends thereof.
In FIG. 1, a configuration of the single cell which forms the fuel cell stack is shown. As shown in FIG. 1, a single cell 100 includes a membrane electrode assembly in which an oxygen electrode 102 and a hydrogen electrode 103 are integrated by being joined to both sides of a polymer electrolyte membrane 101. Each of the oxygen electrode 102 and the hydrogen electrode 103 has a two-layer structure including a reaction membrane 104 and a gas diffusion layer 105. The reaction membranes 104 are in contact with the polymer electrolyte membrane 101. On both sides of the oxygen electrode 102 and the hydrogen electrode 103, an oxygen electrode-side separator 106 and a hydrogen electrode-side separator 107 are placed individually. Moreover, by the oxygen electrode-side separator 106 and the hydrogen electrode-side separator 107, an oxygen gas passage, a hydrogen gas passage, and a coolant passage are formed.
The single cell 100 with the above-described configuration is manufactured in a manner that the oxygen electrode 102 and the hydrogen electrode 103 are placed on both sides of the polymer electrolyte membrane 101, are integrally joined usually by a hot press method to form the membrane electrode assembly, and next, the separators 106 and 107 are placed on both sides of the membrane electrode assembly. In a fuel cell composed of the above-described single cells 100, when mixed gas of hydrogen, nitrogen and steam is supplied to the hydrogen electrode 103 side and the air and steam are supplied to the oxygen electrode 102 side, an electrochemical reaction occurs mainly on contact surfaces of the polymer electrolyte membrane 101 and the reaction membranes 104. A more specific reaction is described below.
In the single cell 100 with the above-described configuration, when oxygen gas and hydrogen gas are supplied to the oxygen gas passage and the hydrogen gas passage, respectively, the oxygen gas and the hydrogen gas are supplied to the reaction membranes 104 through the respective gas diffusion layers 105, and in the respective reaction membranes 104, reactions to be shown below occur.Hydrogen electrode: H2→2H++2e−  (Expression 1)Oxygen electrode: (1/2)O2+2H++2e−→H2O   (Expression 2)
When the hydrogen gas is supplied to the hydrogen electrode 103, the reaction of Expression 1 progresses, and H+ and e− are generated. H+ moves through the inside of the polymer electrolyte membrane 101 and flows into the oxygen electrode 102, and e− flows from the hydrogen electrode 103 to the oxygen electrode 102 through a load 108. In the oxygen electrode 102, the reaction of Expression 2 progresses by H+, e− and the supplied oxygen gas, and then electric power is generated.
As described above, the fuel cell separators, which are used for the fuel cell stack, have a function to electrically connect the respective single cells to each other. Accordingly, it is required for the separators to have good electrical conductivity and low contact resistance with constituent materials such as the gas diffusion layers. With regard to an overvoltage owing to resistance polarization in the fuel cell, in a stationary use, exhaust heat is collected by means of cogeneration and the like, and an improvement of thermal efficiency can be expected as a whole. However, in a use for the vehicle, with regard to a heat generation loss based on the contact resistance, there is no other way but to throw generated heat away from a radiator to the outside through the coolant. Accordingly, a large contact resistance leads to a decrease of the power generation efficiency. Moreover, this decrease of the efficiency is equivalent to an increase of heat generation, and accordingly, a necessity also arises that a larger cooling system should be installed. As described above, a decrease of the contact resistance is an important subject to be solved.
In addition, temperatures of the respective gases supplied to the fuel cell are as high as 80 to 90° C. Moreover, as described above, the hydrogen electrode where H+ is generated and the oxygen electrode where the oxygen, the air and the like pass are in an acidic atmosphere where a degree of acidity (pH) is 2 to 3. Accordingly, for both of the oxygen electrode-side and hydrogen electrode-side separators, corrosion resistance against a strong acidic atmosphere is required. Therefore, as the separators, it is conceived to use stainless steel which has good electrical conductivity and the corrosion resistance. On the stainless steel, a passive film of which surface is dense, that is, chromium oxide (CrO.OH.nH2O, Cr2O3.xH2O) is formed, and accordingly, the stainless steel has excellent corrosion resistance. However, this passive film causes the contact resistance with carbon paper usually used as the gas diffusion layer.
In this connection, among separators formed by press-molding the stainless steel, a separator in which a gold-plated layer is directly formed on a surface contacting the electrode has been proposed (refer to Japanese Patent Laid-Open Publication No. H10-228914). Moreover, a separator has been proposed, in which the stainless steel and titanium are processed into the separator, the passive film on a surface contacting another member to cause the contact resistance is then removed, and noble metal or a noble metal alloy is adhered onto the surface (refer to Japanese Patent Laid-Open Publication No. 2001-6713). Furthermore, a separator has been proposed, in which two or more layers of metal nitrides different in type, such as titanium nitride (TiN) and chromium nitride (CrN), are formed on the surface of the stainless steel (refer to Japanese Patent Laid-Open Publication No. 2001-236967).